This invention generally relates to electric discharge lamps and has particular reference to an improved method and composition for phosphor-coating fluorescent lamps having envelopes which are precoated with a transparent film of conductive material to facilitate lamp starting.
Electric discharge lamps having tubular glass envelopes that are provided with an interior transparent film of various materials which serves as a protective-barrier for the phosphor coating or as a starting aid are generally well known in the art. U.S. Pat. No. 3,067,356 granted Dec. 4, 1962 to Ray, for example, discloses a fluorescent lamp wherein mercury-alkali reaction at the inner surface of the glass envelope is inhibited by a transparent layer of Al.sub.2 0.sub.3, SiO.sub.2, or TiO.sub.2. The use of a transparent layer of titanium dioxide or zirconium dioxide which contains selected metal oxides as additives and forms a protective coating for a fluorescent lamp envelope is disclosed in U.S. Pat. No. 3,377,494 issued Apr. 9, 1968 to Repsher. An aperture-type fluorescent lamp wherein the portion of the glass envelope which defines the aperture is coated with a conducting transparent coating of tin oxide or indium oxide is described in U.S. Pat. No. 3,809,944 issued May 7, 1974 to Jongerius et al. A method of improving the adhesion of a reflecting coating of titanium oxide or aluminum oxide that is applied to the inner surface of a fluorescent lamp bulb which has previously been coated with a transparent electrically conductive coating is disclosed in Japanese Pat. No. 43-25213 of Kobajashi et al. granted Oct. 31, 1968. The method consists of adding nitrates and/or borates of zinc, calcium, cadmium, barium or magnesium in an aqueous solution to the reflector-forming material, together with some ammonia, and then drying the resulting reaction product before it is applied to the bulb.
Various techniques for improving the adherence of phosphor coatings to fluorescent lamp envelopes which do not have transparent coatings are also generally well known in the art. In U.S. Pat. No. 2,905,572 granted Sept. 22, 1959 to Jones, this objective is achieved by adding materials such as calcium nitrate and an ethyl borate solution to the phosphor paint, which materials decompose during the bulb lehring operation to form a Ca0:B.sub.2 O.sub.3 cementing material which bonds the phosphor particles to the untreated glass surface. Improved lamp phosphor adherence by adding from 0.05 to 0.3 weight percent (based on the phosphor content) barium nitrate to the phosphor-containing coating composition applied to conventional glass bulbs (no transparent film) is disclosed in U.S. Pat. No. 3,424,605 issued Jan. 28, 1969 to Beaumont et al. U.S. Pat. No. 3,424,606 to Giudici effects a further improvement in phosphor adherence by including from 0.1 to 2.5 weight percent ammonium nitrate, in combination with 0.05 to 0.3 weight percent barium nitrate, in an aqueous suspension of the phosphor and an organic binder that is applied to such conventional bulbs.
U.S. Pat. No. 3,585,207 issued June 15, 1971 to Repsher discloses a water-base phosphor-coating composition wherein improved adhesion of the lehred phosphor layer to a conventional lamp bulb (no film) is obtained by adding to the coating composition from 0.1 to 1.0 weight percent of aluminum oxide (in the form of an Alon-C material which consists of finely-divided aluminum oxide particles of a specified size range). U.S. Pat. No. 3,833,398 to Schreurs discloses another method of improving the adherence of the phosphor coating in a conventional type fluorescent lamp by pretreating the phosphor and aluminum oxide particles with a solution of ammonium lignosulfonate.
In order to reduce the starting voltage of fluorescent lamps which contain a mixed fill gas (such as krypton and neon) that decreases the wattage loading of the lamps, it has become the practice to coat the inner surface of the glass bulb of such lamps with a transparent film of conductive material before the phosphor coating is applied. The transparent film not only makes it very difficult to bond the phosphor particles to the bulb surface but requires that lower lehring temperatures be employed to prevent an undesirable decrease in the conductivity of the film. Such lower lehring temperatures (in the order of 580.degree. C. versus the 650.degree. C. or so generally used) further aggravates the phosphor-adherence problem. In an effort to solve this problem, the Alon-C additive to the phosphor coating composition has been increased (up to 1.5 weight %) but this results in a drop in the light output of the lamps.
It has been discovered that the poor adherence of the phosphor coating experienced in the production of fluorescent lamps having envelopes which are provided with such transparent conductive films (tin oxide, for example) can be prevented by employing selected and very precisely correlated amounts of aluminum oxide particles (Alon-C), in combination with ammonium nitrate and a third additive selected from the group consisting of calcium nitrate, barium nitrate, cadmium nitrate, strontium nitrate and mixtures thereof. These materials are added to the aqueous coating composition or "paint" which contains the phosphor particles, a suitable organic binder and small amounts of defoaming and wetting agents. By virtue of the manner in which the quantities of the various additives are correlated, such additives not only reduce the amount of aluminum oxide (Alon-C additive) heretofore employed in the phosphor paint but permit adherent phosphor coatings to be produced despite the lower lehring temperatures which are required to prevent undesirable decreases in the conductivity of the transparent film.
Improved phosphor adhesion and enhanced light output have been achieved in the case of tin-oxide coated fluorescent lamps by using only from about 0.1 to 0.6 weight percent of Alon-C in the aqueous phosphor-coating composition in combination with from about 0.05 to 0.25 weight percent calcium nitrate and from about 0.1 to 0.8 weight percent ammonium nitrate. Alternatively, preliminary tests have also indicated that satisfactory phosphor adhesion can be achieved in such lamps by omitting the ammonium nitrate additive and simply mixing selected amounts of two of the other nitrate additives (for example calcium nitrate and barium nitrate) which together form a dual-component "eutectic like" constituent that has a lower melting point than either of the components if used separately.